1. Field of the Invention
The present invention relates generally to a delta-sigma (.DELTA..SIGMA.) type analog-to-digital converter, and more particularly to removal of a DC component in a .DELTA..SIGMA. type analog-to-digital converter.
2. Description of Related Art
Prior art approaches for removing a DC component or offset employed in conventional .DELTA..SIGMA. type analog-to-digital converters (A/D converters) may be roughly classified into two types: an offset calibration method and a method utilizing a high pass filter. An example of the offset calibration method is disclosed in U.S. Pat. No. 4,943,807 in which an offset value of a .DELTA..SIGMA. A/D converter is stored in a memory, and subtracted from a digitally converted value (a value after passing through a .DELTA..SIGMA. modulator and a decimation filter) to remove the DC component. The second approach utilizing a high pass filter passes a digitally converted value through a high pass filter to remove a DC component possibly included in the digitally converted value.
In the offset calibration method, if certain external factors or the like cause an offset value to change during a long time operation of an A/D converter, a difference between a value initially stored in a memory and the changed offset value is produced, thereby preventing complete removal of an associated DC component. The high pass filter approach, on the other hand, is effective to remove an offset varying during operation of an A/D converter as well. However, since the high pass filter approach utilizes a filter having an extremely low cut-off frequency which is characteristic required to remove a DC component, it takes a long time to remove the first DC component from the start of operation of the filter. Such a removal time, in fact, occupies a substantial portion within a total time required from power-on of the A/D converter to output of a stable correct value.